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It is assur<forbidden> to listen to a woman sing, since Kol B'Isha Erva<the voice of a woman is nakedness>. What about listening to her playing a musical instrument, is that considered Kol Isha? Rav Menashe Klein says (Mishneh Halachos 6:25) that it is not, and only her actual voice is an erva<sexual part>.

However says Rav Klein, while Kol Isha is not a problem there are other issurim involved that are quite possibly severe. The gemara <rabbinical commentary> (Eiruvin 18b) says that a person may not walk behind a woman even his own wife. The Radvaz (770) says that this applies even if she is dressed in a perfectly modest manner. The reason is that by watching her body movements you are inciting the Yetzer Hara<evil inclination>, which will cause you harm. The forbidden distance is not limited to four amos<approx. 8 feet>, but any distance where you can make out her movements.

Rav Klein says that we see from here that it is assur to focus on the movements of a woman, even your own wife and certainly if she is a niddah< menstruating woman>. When it comes to watching a woman play an instrument where you are mesmerized by her movements and her performance, although it is not Kol Isha, it is certainly assur and maybe even more severe than Kol Isha, and this goes into the category of Abizraihu D'Giluy Arayos<forbidden sexual relations>.